In a long-term evolution (LTE) compliant wireless communication system, a physical random access channel (PRACH) is used by a user equipment (UE) to initiate network access by sending a PRACH signal over the PRACH to a base station, known in the LTE literature as an eNodeB (Evolved Node B). The PRACH signal contains a preamble, which is a signature sequence selected from a set of sequences according to the specification of the LTE compliant wireless communication system. Each of such sequences has a unique preamble index. Details regarding the preamble and the preamble index thereof can be found in S. Sesia, I. Toufik and M. Baker (ed.), LTE, The UMTS Long Term Evolution: From Theory to Practice, John Wiley & Sons, 2009, the disclosure of which is incorporated by reference in its entirety herein. The base station is required to detect arrival of the PRACH signal by detecting presence or absence of the preamble. Furthermore, the base station is required to determine the preamble index of the preamble and report some measurements to upper layer.
In an uplink of the LTE compliant system, certain subframes are configured with PRACHs, on which PRACH signals sent from UEs can be transmitted. In a subframe having a PRACH, the PRACH is frequency-multiplexed with other uplink channels, namely, physical uplink shared channels (PUSCHs) and physical uplink control channels (PUCCHs), to form an uplink signal. In general, such uplink signal may include: no PRACH signal at all, or one PRACH signal sent from one UE, or a number of PRACH signals sent from multiple UEs; and one or more other-channel signals from other UEs on the PUSCHs and/or the PUCCHs. In particular, these other UEs send the other-channel signals in a time-synchronized manner. However, since different UEs have different distances from the base station, signals sent from these other UEs arrive at the base station with slightly different time delays. Hence, the received uplink signal is such that boundaries of transmitted symbols present in the other-channel signals on the PUSCHs and/or the PUCCHs are approximately time-aligned. To detect presence or absence of a possible preamble signature sequence in the PRACH, the receiver is required to perform such detection in the presence of interference caused by the other-channel signals. The detection problem is further complicated in that a UE prefers to send a PRACH signal with minimal power just sufficient enough for the preamble index carried in the preamble to be detected while UEs send the other-channel signals often with high transmit power levels in order to achieve high throughput (i.e. high spectral efficiency).
An important consideration in processing the uplink signal received by the base station for detecting a preamble signature sequence is to minimize the false alarm rate. While US20100150277 and WO2011120255 address the problems of reducing the false alarm rates due to the effect of multipath dispersion and due to the carrier frequency offset, respectively, it is also evident that minimizing the false alarm rate in the presence of other-channel signals having transmit power higher than that of the PRACH signal has practical applications. Filtering, which is used to extract the PRACH signal from the uplink signal, also reduces power levels of the other-channel signals. However, in some situations using filtering techniques alone is not adequate when the power level of the sum of the other-channel signals is substantially higher than that of the PRACH signal. For example, the difference in the power level may be as high as 35 dB.
It is desirable to have additional techniques that can work with the filtering techniques for further reducing the false alarm rate in the presence of the other-channel signals. There is a need in the art for such additional techniques.